I mean that every target presents some kind of resistance, be it a wooden door or a human body or a pile of bricks. I believe the amount of power put in a strike is proportional to the amount of resistance the attacker expects to receive. When the encountered resistance is nil, there must be some kind of loss of balance even by the most experienced martial artists.

A Karateka can hit the air while performing a Kata with great power without loosing balance because he did not expect resistance, but the same Karateka if he's trying to break a pile of bricks will surely loose his balance if the bricks were to suddenly disappear just before the moment of impact.

But I could be wrong.

It really depends on the attack. Such loss of balance could be useful in a setup. This is why many arts such as mauy thai and boxing throw combos instead of the one shot one kill mentality in many asian arts. For example, I could throw a hard leg kick and miss, but you would be careful to enter because a back fist might be coming your way. Another example is I might throw a less powerful leg kick to setup a punch such as an overhand right. I do not care if the kick lands or not, it has enough power to cause some minor damage, but its real purpose is to allow me to enter into punching range while keeping your strikes at bay.

Another example would be in judo. I grab to pull. Every grab is a pull. I have been told by a few japanese jj guys and aikido guys that grabs are pushes, but I really feel they are pulls. If you move I lose no balance. If you enter I lose no balance, and I am fully prepared to move with you and continue to attempt to take your balance. This is because I am not reaching out to make the grab. I move my body to the proper distance to make the grab work. This is where a good strike could upset my balance. I grab you step in with the pull preventing your loss of balance. As you step in you strike to my face. Had you not struck I would probably adjust with no effort and continue my attempt to break balance and throw. But with the strike I can not continue my grab and protect my face. I have to choose one. My will allow you a chance to break my balance, either though striking my skull, or a technique based on my raising the hands to block and moving my head. Of course you will need to keep moving as I circle out of there.

Again this is chessboard martial arts, a way of working I hate. I prefer to make my points by encouraging sparing. But the main point I want to make is that not every strike is designed or meant to break a brick. A jab doesn't have to knock your block off to be very effective, it just has to make the opening you need for that big shot.